Electrophotographic photoreceptors containing organic photoconductive materials have been extensively studied because such photoreceptors do not cause environmental pollution, have high productivity and are inexpensive. There are known electrophotographic photoreceptors wherein the photosensitive layer contains diphenyldicyanoethylene derivatives as the sensitizing agent. Such a photoreceptor is disclosed in JP-A-54-30834 (the term "JP-A" refers to an unexamined published Japanese patent application).
However, a problem with such use of organic photoconductive materials is that materials which absorb visible light and generate electric charges exhibit poor charge retention characteristics; whereas materials having good charge retention characteristics and excellent film forming properties barely exhibit photoconductivity by visible light. In order to solve these problems, the prior art used a photosensitive layer of a laminate type composed of a layer containing a charge generating agent which absorbs visible light and generates electric charges and another layer containing a charge transporting agent which transports the charges produced. Various charge transporting agents have been proposed. For example, when the charge transporting agent is of the positive hole transporting agent type, amine compounds, hydrazone compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, stilbene compounds and carbazole compounds can be used. Also, the other charge transporting agent, i.e., of the electron transporting agent type is 2,4,7-trinitrofluorenone (TNF) for example. Other examples of charge transporting agents include compounds containing boron described in JP-B-48-9988 (the term "JP-B" refers to an examined Japanese patent publication) and Canadian Patent 912,019.
As yet, no completely satisfactory sensitizing agent has been found for use in single layer structure type electrophotographic photoreceptors using an organic photoconductive material. For electrophotographic photoreceptors having a laminated structure, a positively charging type is preferred from the viewpoints of preventing ozone generation due to corotron and of controlling the charging of the toner in development. When the laminated type layer electrophotographic photoreceptor is of the positively charging type and when the charge transporting agent is of the positive hole transporting type, it is necessary that the charge generating layer be the upper layer. However, because the custom is to make the charge generating layer thin, the problem arises that such a thin charge generating layer is insufficiently thick to meet the mechanical requirements of the photoreceptor. Thus, it is imperative to use a positively charging photoreceptor with a sufficiently thick charge transporting layer as the upper layer. To provide such a desired photoreceptor, it is necessary to use an electron transporting agent in the charge transporting layer. However, the prior art charge transporting agents are not sufficiently effective as electron transporting agents.
The present invention is intended to overcome the above noted problems associated with the prior art.